Method for producing a chromium activated aluminum oxide scintillator

ABSTRACT

A RADIATION RESISTANT CHROMIUM ACTIVATED ALUMINUM OXIDE SCINTILLATOR PRODUCED BY IMMERSING AN ALUMINUM ANODE AND LEAD CATHODE IN AN ANODIC BATH AND GRADUALLY INCREASING AN APPLIED POTENTIAL UNTIL BRIGHT FLASHES OR &#34;ANODIC SCINTILLATION&#34; OCCURS WHEREBY CR IONS FROM THE SOLUTION ARE INJECTED INTO HE AL2O3 BASE LATTICE. THE SCINTILLATORS PRODUCED BY THIS PROCESS MAY BE READILY UTILIZED TO VISUALLY AID IN THE CONTROL AND TRANSPORT OF HIGH INTENSITY BEAMS WITHOUT RADIATION DAMAGE THERETO.

May 25, 1971 w, L JR" ETAL 3,580,822

METHOD FOR PRODUCING A CHROMIUM ACTIVATED ALUMINUM OXIDE SCINTILLATOR Filed July 7, 1969 24 SPECTROMETER lji /lllllmllllll/ INVENTORS ROBERT W ALL/SON JR. ROBERT W. BROKLOFF BY JOHN R. WOODYARD United States Patent O METHOD FOR PRODUCING A CIIROMIUM ACTI- VATED ALUMINUM OXIDE SCINTILLATOR Robert W. Allison, Jr., Richmond, Robert W. Broklofl,

Vallejo, and John R. Woodyard, Berkeley, Callf., assignors to the United States of America as represented by the United States Atomic Energy Commission Filed July 7, 1969, Ser. No. 839,510 Int. Cl. C23b 9/02 U.S. Cl. 204-58 7 Claims ABSTRACT OF THE DISCLOSURE A radiation resistant chromium activated aluminum oxide scintillator produced by immersing an aluminum anode and lead cathode in an anodic bath and gradually increasing an applied potential until bright flashes or anodic scintillation occurs whereby Cr ions from the solution are injected into the A1 base lattice. The scintillators produced by this process may be readily utilized to visually aid in the control and transport of high intensity beams without radiation damage thereto.

BACKGROUND OF THE INVENTION The invention described herein was made in the course of, or under, Contract No. W-7405-ENG-48, with the United States Atomic Energy Commission.

One of the most striking features of accelerator technology today is the production of intense beams of particles. Such beams with currents varying from milliamperes to tens of amperes pulsed make it possible to perform many simultaneous experiments. But the control and transport of intense beams require new diagnostic devices. Scintillators of zinc-sulfide or activated plastic have been standard diagnostic devices in many medium intensity beams. These prior known scintillators are susceptible to radiation damage and when used in intense beams must be frequently replaced.

SUMMARY OF THE INVENTION The scintillators produced in accordance with the present invention overcomes the problems of the prior art in that they are resistant to radiation damage and thus can be readily utilized in high intensity beams without frequent replacement and damage thereto. The scintillator of the present invention is produced by immersing an aluminum anode and lead cathode in an anodic bath and gradually increasing an applied potential until bright flashes or anodic scintillation occurs depositing traces of chromium in the aluminum oxide.

Therefore, it is an object of this invention to provide an improved charged particle radiation detector for high intensity beams and method of producing same.

A further object of the invention is to provide a method for producing a scintillator which is relatively immune to damage from a high intensity charge particle beam and which may be used for visual observation of beam position.

A further object of the invention is to provide a method for producing an aluminum oxide-chromium activated scintillator.

' Other objects of the invention will become readily apparent from the following description and accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING The single figure illustrates an embodiment of an apparatus for carrying out the novel method of the invention.

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DESCRIPTION OF THE INVENTION The method for forming aluminum oxide-chromium activated scintillators which are relatively immune to damage from a high intensity charged particle beam and which may be utilized to visually aid in the control and transport of such beams is exemplified as follows:

The scintillators were prepared in an anodic bath. The solution used is: 788 grams of anhydrous sodium borate dissolved in 11.5 gallons of water, low conductivity being used, to which is added ml. of saturated potassium dichromate as a doping agent. The anodizing tank is of PVC plastic. The cell cathode is a plate of 99.8% pure electrolytic grade lead. The anode is a pure aluminum plate 4.5 inches square, 0.025 inch thick. This plate is mounted with silicone grease on a water cooled copper back and inserted into a Lucite holder which shields the anode so that only the front surface of the aluminum plate comes in contact with the solution. The solution is cooled by circulating it through a water chiller or heat exchanger. A stainless steel pump is used to avoid contamination. The pump also sets up turbulence within the tank which mixes the solution during the anodizing process. A temperature of 10 C. to 12 C. is held during each anodizing run. The apparatus for carrying out the inventive method will be described following the description of the novel process to provide a clearer understanding of an embodiment thereof for producing the scintillator.

Anodizing is done at a constant current of 7 amperes. When the process is first started a tank voltage of 50 to 100 volts is observed. As the oxide film forms, a blue luminescence is observed at voltages from 100 to 600 volts. As the film thickens the voltage required to maintain a constant current increases. At approximately 600 volts a phenomenon which limits the voltage occurs and is described herein as anodic scintillation. Because the anodic film is an insulator most of the applied voltage appears across the film and very high internal electric fields occur. When a sufficiently high field is reached the film is punctured Which results in a brilliant flash and occasionally an audible pop. During this anodic scintillation Cr ions are formed and injected into the A1 0 base lattice by the electric field, the Cr ions being obtained from the (Cr- O cations of the anodic bath. The exact mechanism is not understood, but anodic scintillation as described above is required for activation. The amount of activation is proportional to the number of ampere-hours run. A minimum of 5 ampere hours/in. are required to form a useful scintillator. The amount of activation is observed with a spectroscope, the intensity of the CT 3d lines at 6900 A. being proportional to the amount of chromium activator injected into the base A1 0 lattice.

The scintillators made in accordance with the above described process are subjected to what is herein termed luminescence scintillation during their use in high intensity particle beams for visually aiding in the control and transport of such beams.

Referring now to an embodiment of an apparatus for carrying out the above described method, the single figure illustrates such to an extent necessary to provide a full understanding thereof. As illustrated, an open top anodizing tank or vessel 10, constructed, for example, of PVC plastic, is mounted on a support member 11, provided in one wall thereof with a Lucite window 12, and retains therein an anodic bath (doped borate solution) as described above. The solution within tank 10 is cooled by circulating it through a heat exchanger 13 by a pump 14 as indicated by the flow arrows through conduits 15, 16 and 17 and a distributor 18, conduit 15 being secured to an opening in the lower portion of one wall of tank 10 while distributor 18 extends into the top of the tank and supported thereon by a holder or support member 19 which may be made of Lucite or other suitable material. As conventional, heat exchanger 13 is supplied with cooling fluid, such as chilled water, via conduits 20 and 21. The conduits 15-17 and distributor 18 may, for example, be constructed of PVC plastic, and as pointed out above, the pump 14 may be stainless steel to prevent contamination, with heat exchanger 13 being constructed of material compatible with the borate solution. A light shield 22 is mounted in one wall of tank 10 with a light pipe 23 secured therein, light pipe 23 terminating at the outer end thereof in a spectrometer 24 which functions as known in the art for detecting, through light pipe 23, any illumination within tank 10. Positioned on the upper edge of two of the walls of tank 10 are Lucite support strips 25 and 26, support member 19 being mounted on support strip 25,, while each of strips 25 and 26 are provided with a notch or slot 27 for supporting a conductive bar 28 of a cathode assembly generally indicated at 29, assembly 29 being shown in a removed position with respect to notches 27 for clarity. In addition to bar 28 cathode assembly 29 includes a clamp mechanism 30 and a cathode element or electrode 31 composed of a plate or sheet of lead as described above which is suspended in the doped borate solution in tank 10, bar 28 being electrically connected to the negative terminal of a direct current power supply 32 of the 2 kv. 7 amp type, for example. A pair of Lucite support elements 33 and 34 are respectively mounted on support strips 25 and 26 and are each provided with a notch or slot 35 for supporting a conductive bar 36 of an anode assembly generally indicated at 37, anode assembly 37 being shown removed from notches 35 for clarity. In addition to bar 36 anode assembly 37 includes a clamp mechanism 38 securing bar 36 to a Lucite holder 39 within which is mounted an anode element or electrode 40 composed of a foil, plate or sheet of aluminum as described above. While not shown, but as described above, aluminum anode plate 40 is mounted with silicon grease on a water cooled copper back and inserted into the holder 39 which shields the anode plate so that only the front surface, as shown, of the aluminum plate 40 comes in contact with the borate solution in tank 10 when bar 36 is positioned in notches 35 of support elements 33 and 34. Anode bar 36 is electrically connected to the positive terminal of the power supply 32, whereby upon insertion of the anode and cathode elements 31 and 40 into the solution in tank 10 and with power supply 32 activated an electrical circuit is completed through the solution as common in the art, and as described above, as the oxide film forms an aluminum plate 40, the voltage is increased to maintain a constant current of 7 amperes, in this embodiment, until the phenomenon of anodic scintillation occurs whereupon Cr ions from the chromium doped borate solution are injected into the A1 base lattice of anode plate 40, the brilliant flashes of such anodic scintillation being detccted by the spectrometer 24 through light pipe 23 and visually observed through Lucite window 12 in tank 10. As described above the temperature of the anodizing solution is maintained as desired by the circulation of the solution through the heat exchanger 13 by pump 14. The activation period, as above described, is a minimum of ampere hours/in. to form a useful scintillator for use in high intensity particle beams. Again, while the exact mechanism or theory of the inventive process is not fully understood, but tests on scintillators made in accordance with the invention as exemplified hereinafter, have proven the inventive concept to provide a scintillator which greatly advances the state of the art of diagnostic devices for intense beams of particles in the accelerator technology.

Tests in a 3.4 mev. electron beam and in a 5 gev. external proton beam described in detail hereinbelow indicate that scintillators made in accordance with the present invention are sensitive enough to be useful and that they have a high resistance to radiation.

An inventive scintillator has been tested in the Astron injector 3.4 mev. electron beam. The luminescence (luminescence scintillation) was found to be more than adequate and the scintillator was used with a slit-plate to observe the emittance of the beam of each pulse. A peak current of 250 amperes for 300 nanoseconds was run. The pulse repetition frequency was 1 to 2 pulses per second. The scintillator received about pulses of full intensity (250 A.) and ran for 15 to 20 hours with the slit-plate in the beam, at an intensity of 10 amperes. At the conclusion of the test there was no discoloration and no decrease in luminosity was noted.

A scintillator was also tested in the Bevatron external proton beam. The beam energy was 5 gev. A 700 microsecond spill of l 10 particles was clearly visible. Al though the visible threshold of the inventive scintillators have not been accurately determined, it is exposed that beams of 1x 10 particles/cmP-sec. will be observable. A 24 hour ran with a particle spill length of 200 msec. and intensity of 5 X10" protons/ pulse produced no observable decrease in luminosity. The pulse repetition frequency was 10.9 pulses/min.

In both of the above tests the luminescence obtained was from excitation of the Cr ions and had two dominant sharp line spectra at 6928 and 6942 angstroms.

The above tests have thus shown that the present invention provides a method for producing radiation resistant scintillators for use in intense particle beams.

It has thus been shown that aluminum oxide is an excellent material because of its luminescent properties, strength, and radiation resistance with chromium ions obtained from the doping agent in the solution and injected into the base lattice by anodic scintillation. Spectroscopic observation of the activator excitation during anodic scintillation permits control of the scintillator sensitivity. The high power used in anodization and the temperature control achieved during the inventive process make it possible to form more regular films than previously attained. Thus, as a result of the present inventive method, scintillators can be readily produced for uses such as diagnostic devices for the control and transport of intense beams.

We claim:

1. A method for producing radiation resistant chromium activated aluminum oxide scintillators comprising the steps of: providing a chromium containing anodizing bath solution; maintaining the solution at a predetermined temperature; positioning at least one aluminum anode means and at least one lead cathode means in the solution in spaced relationship; anodizing by applying an electrical potential to provide a current between the anode and cathode means through the solution to initiate the formation of an oxide film on the aluminum anode means; and maintaining the current substantially constant by increasing the potential as the film thickness increases until the potential is limited by an anodic scintillation phenomenon wherein chromium ions from the solution are injected into the aluminum oxide base lattice of the anode means.

2. The method defined in claim 1, wherein the anodizing bath solution consists essentially of 788 grams of anhydrous sodium borate and 100 milliliters of saturated potassium dichromate per 11.5 gallons of water.

3. The method defined in claim 1, additionally including the steps of: preparing the aluminum anode means from a pure aluminum plate, mounting the plate with silicone grease on a water cooled copper back, and inserting the thus backed plate into a Lucite holder such that only the front surface of the aluminum plate comes in contact with the solution; and preparing the lead cathode means from a plate of 99.8% pure electrolytic grade lead.

4. The method defined in claim 1, wherein the step of maintaining the solution at a predetermined temperature is accomplished by circulating the solution through a cooler means capable of maintaining the solution at a temperature in the range of about 10 C. to 12 C.

5. The method defined in claim 1, wherein the current is maintained substantially constant at about 7 amperes.

6. The method defined in claim 1, wherein the applied potential for initiating the anodizing operation is in the range of about 50-100 volts, and the potential is increased to about 600 volts to maintain the current constant.

7. The method defined in claim 1, wherein the current is passed through the solution at the limit of the electrical potential for a minimum of 5 ampere hours per square inch based on the area of the anode means.

References Cited UNITED STATES PATENTS 3,293,158 12/1966 McNeill et al 204-56 2,346,658 4/1944 Brennan et a1. 20458 HOWARD s. WILLIAMS, Primary Examiner R. L. ANDREWS, Assistant Examiner 

